Oxidation process for the production of alkenes and carboxylic acids

ABSTRACT

A process for the oxidation of a C 2  to C 4  alkane to produce the corresponding alkene and carboxylic acid which process comprises contacting in an oxidation reaction zone, said alkane, molecular oxygen-containing gas, and optionally, at least one of the corresponding alkene and water, in the presence of at least two catalysts each active, with different selectivities, for the oxidation of the alkane to the corresponding alkene and carboxylic acid, to produce a product stream comprising said alkene, carboxylic acid and water, and in which process the molar ratio of alkene to carboxylic acid produced in said oxidation reaction zone is adjusted or maintained at a pre-determined value by controlling the relative proportions of the at least two catalysts in said oxidation reaction zone. Such an oxidation process may be used in an integrated process, such as for the manufacture of ethyl acetate or vinyl acetate.

[0001] The present invention relates to a process for the oxidation of aC₂ to C₄ alkane to produce the corresponding alkene and carboxylic acidand to integrated processes in which the alkene and carboxylic acid arefurther used as reactants.

[0002] Carboxylic acids are useful feedstocks for the production ofalkenyl carboxylates. Thus, for example, acetic acid is used tomanufacture vinyl acetate which is generally prepared commercially bycontacting ethylene and acetic acid with molecular oxygen in thepresence of a catalyst active for the production of vinyl acetate.Suitably, the catalyst may comprise palladium, an alkali metal acetatepromoter and an optional co-promoter (for example, gold or cadmium) on acatalyst support. Acetic acid may be produced by the catalytic oxidationof ethylene and/or ethane.

[0003] Integrated processes for producing acetic acid and/or vinylacetate are known in the art. EP-A-0 877 727 discloses an integratedprocess for the production of acetic acid and/or vinyl acetate in anypre-determined and variable proportions from a gaseous feedstockcomprising ethylene and/or ethane. The integrated process comprises afirst step wherein ethylene and/or ethane is catalytically oxidised in afirst reaction zone to produce a first product stream comprising aceticacid, water and ethylene and optionally ethane, carbon monoxide, carbondioxide and/or nitrogen. The acetic acid and ethylene produced in thisfirst reaction zone are then contacted in a second reaction zone with amolecular oxygen-containing gas in the presence of a catalyst to producea second product stream comprising vinyl acetate, water, acetic acid andoptionally ethylene. No mention is made of any control of the productionratio of ethylene to acetic acid from the catalytic oxidation of ethaneand/or ethylene.

[0004] Research Disclosure 2244 of 1992(June) No. 338 describes aprocess for the oxidation of ethane and/or ethylene to produce aceticacid in which the by-product carbon monoxide is oxidised to carbondioxide. According to this document, the acetic acid, unreacted ethane(if present) and ethylene is passed with or without carbon dioxide andwater removal, to a reactor having a suitable catalyst for theproduction of ethyl acetate or, with the addition of oxygen, for theproduction of vinyl acetate. This document is silent on the control ofthe ratio of ethylene to acetic acid produced in the oxidation step.

[0005] In the manufacture of vinyl acetate from ethylene and aceticacid, the molar ratio of the fresh feed ethylene to acetic acid isdesirably unity or approximately unity. Thus, in an integrated processin which ethane is oxidised in an oxidation reaction zone to produceethylene and acetic acid for use in a second reaction zone for theproduction of vinyl acetate, to maximise overall integrated processefficiency and also vinyl acetate output, the molar ratio of ethylene toacetic acid produced in the oxidation reaction zone, is desirably unityor approximately unity depending upon the selectivity/yield in thesecond reaction zone.

[0006] Thus, there remains a need for a process for the oxidation of aC₂ to C₄ alkane to produce the corresponding alkene and carboxylic acidin which the molar ratio of alkene to carboxylic acid produced isadjusted or maintained at a pre-determined value.

[0007] Accordingly, the present invention provides a process for theoxidation of a C₂ to C₄ alkane to produce the corresponding alkene andcarboxylic acid which process comprises contacting in an oxidationreaction zone, said alkane, a molecular oxygen-containing gas andoptionally, at least one of the corresponding alkene and water, in thepresence of at least two catalysts each active, with differentselectivities, for the oxidation of the alkane to the correspondingalkene and carboxylic acid, to produce a product stream comprisingalkene, carboxylic acid and water, and in which process the molar ratioof alkene to carboxylic acid produced in said oxidation reaction zone isadjusted or maintained at a pre-determined value by controlling therelative proportions of the at least two catalysts in said oxidationreaction zone.

[0008] Each of the alkane, molecular oxygen-containing gas, alkene andwater may be introduced into the oxidation reaction zone as fresh feedand/or recycle component.

[0009] The selectivity of a catalyst for alkene or carboxylic acid isdefined as the proportion of reactant which is converted to alkene orcarboxylic acid by that catalyst.

[0010] If one or more of the catalysts in the oxidation reaction zonede-activate in use and it is necessary to replace catalyst during theprocess, the molar ratio of alkene to carboxylic acid produced may bemaintained at a constant, pre-determined value by controlling therelative proportions of the catalysts in the oxidation reaction zone.Thus, if the activity and/or selectivity of the catalysts in theoxidation reaction zone change independently during the process, thismay involve replacing at least part of the catalysts in the oxidationreaction zone by introducing catalysts into the oxidation reaction zonein different proportions to the proportions of catalysts already in theoxidation reaction zone, in order to maintain the molar ratio of alkeneto carboxylic acid produced. Conversely, if the catalysts in the reactordeactivate such that their individual selectivities do not change, itmay be possible to maintain the molar ratio of alkene to carboxylic acidproduced by using for the replacement of catalysts in the oxidationreaction zone, catalysts in the same proportions as in the oxidationreaction zone.

[0011] The present invention also provides a method of adjusting themolar ratio of alkene to carboxylic acid produced, for example inresponse to changes in demand or requirement in down-stream processes bycontrolling the relative proportions of the at least two catalyst in theoxidation reaction zone.

[0012] The process of the present invention is particularly useful whenthe alkene and/or carboxylic acid products are used at least in part inintegrated downstream processes, for example (a) for the production ofester by reacting the carboxylic acid with the alkene or an alcohol or(b) for the production of alkenyl carboxylate by the reaction of amolecular oxygen-containing gas with the carboxylic acid and alkene.Alkene and/or carboxylic acid may be recovered from the product of theoxidation reaction zone and/or additional alkene and/or carboxylic acidmay be used in the down-stream process.

[0013] In a further embodiment of the present invention, the alkene andcarboxylic acid may be produced in a molar ratio suitable for use in anintegrated down-stream process, for example (a) for the production ofester by reacting the carboxylic acid with the alkene or (b) for theproduction of alkenyl carboxylate by the reaction of a molecularoxygen-containing gas with the carboxylic acid and alkene. If alkeneand/or carboxylic acid is neither separately recovered from reactionproduct nor separately added to the downstream process, the molar ratioof alkene to carboxylic acid produced in the oxidation reaction zone issuitably approximately 1:1, for example, 0.8:1 to 1.4:1. A differentratio may be produced if alkene and/or carboxylic acid is separatelyrecovered from the oxidation reaction product or separately added to thedown-stream process. The molar ratio of alkene to carboxylic acid maythen be adjusted by controlling the relative proportions of the at leasttwo catalysts in the oxidation reaction zone, for example to meetchanges in market demand or feedstock availability. Suitably, the molarratio of alkene to carboxylic acid produced in the oxidation reactionzone is in the range 1:10 to 10:1.

[0014] Accordingly, the present invention provides an integrated processfor the production of an alkyl carboxylate which process comprises thesteps:

[0015] (a) contacting in an oxidation reaction zone a C₂ to C₄ alkane, amolecular oxygen-containing gas and optionally, at least one of thecorresponding alkene and water in the presence of at least two catalystseach active, with different selectivities, for the oxidation of thealkane to the corresponding alkene and carboxylic acid, to produce aproduct stream comprising alkene, carboxylic acid and water; and

[0016] (b) contacting in a second reaction zone at least a portion ofeach of said alkene and carboxylic acid produced in the first reactionzone, in the presence of at least one catalyst active for the productionof alkyl carboxylate to produce said alkyl carboxylate, and in which themolar ratio of alkene to carboxylic acid produced in the oxidationreaction zone is adjusted or maintained at a pre-determined value bycontrolling the relative proportions of the at least two catalysts insaid oxidation reaction zone.

[0017] Also, in another embodiment, the present invention provides anintegrated process for the production of an alkenyl carboxylate whichprocess comprises the steps:

[0018] (a) contacting in an oxidation reaction zone a C₂ to C₄ alkane, amolecular oxygen-containing gas and optionally, at least one of thecorresponding alkene and water in the presence of at least two catalystseach active, with different selectivities, for the oxidation of thealkane to the corresponding alkene and carboxylic acid, to produce aproduct stream comprising alkene, carboxylic acid and water; and

[0019] (b) contacting in a second reaction zone at least a portion ofeach of said alkene and carboxylic acid produced in the first reactionzone and a molecular oxygen-containing gas, in the presence of at leastone catalyst active for the production of alkenyl carboxylate to producesaid alkenyl carboxylate,

[0020] and in which the molar ratio of alkene to carboxylic acidproduced in the oxidation reaction zone is adjusted or maintained at apre-determined value by controlling the relative proportions of the atleast two catalysts in said oxidation reaction zone.

[0021] Preferably, the molar ratio of alkene: carboxylic acid producedin the oxidation reaction zone is maintained at approximately 1:1, forexample, 0.8:1 to 1.4:1 for subsequent use in a second reaction zone forthe production of alkyl carboxylate or alkenyl carboxylate.

[0022] In the present invention, preferably, the C2 to C4 alkane isethane, the corresponding alkene being ethylene and the correspondingcarboxylic acid being acetic acid. These products may be reacted indown-stream processes to produce ethyl acetate or, with a molecularoxygen-containing gas to produce vinyl acetate.

[0023] Typically, the oxidation reaction is performed heterogeneouslywith solid catalysts and the reactants in the fluid phase Catalystsactive for the oxidation of alkane to alkene and carboxylic acid maycomprise any suitable catalysts known in the art, for example, for theoxidation of ethane to ethylene and acetic acid as described in U.S.Pat. No. 4,596,787, EP-A-0407091, DE 19620542, WO 99/20592, DE 19630832,WO 98/47850, WO 99/51339, EP-A-0 1043064, WO 9913980, U.S. Pat. Nos.5,300,682 and 5,300,684, the contents of which are hereby incorporatedby reference.

[0024] U.S. Pat. No. 4,596,787 relates to a process for the lowtemperature oxydehydrogenation of ethane to ethylene using a catalysthaving the empirical formula Mo_(a)V_(b)Nb_(c)Sb_(d)X_(e) as thereindefined, the elements being present in combination with oxygen.

[0025] EP-A-0407091 relates to process and catalyst for the productionof ethylene and/or acetic acid by oxidation of ethane and/or ethylene inthe presence of an oxidation catalyst comprising molybdenum, rhenium andtungsten.

[0026] DE 19620542 relates to molybdenum, palladium, rhenium basedoxidation catalysts for the production of acetic acid from ethane and/orethylene.

[0027] WO 99/20592 relates to a method of selectively producing aceticacid from ethane, ethylene or mixtures thereof and oxygen at hightemperature in the presence of a catalyst having the formulaMO_(a)Pd_(b)X_(c)Y_(d) wherein X represents one or several of Cr, Mn,Nb, Ta, Ti, V, Te and W; Y represents one or several of B, Al, Ga, In,Pt, Zn, Cd, Bi, Ce, Co, Rh, Ir, Cu, Ag, Au, Fe, Ru, Os, K, Rb, Cs, Mg,Ca, Sr, Ba, Nb, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, Tl and U and a=1,b=0.0001 to 0.01, c=0.4 to 1 and d=0.005 to 1.

[0028] German patent application DE 196 30 832 A1 relates to a similarcatalyst composition in which a=1, b>0, c>0 and d=0 to 2. Preferably,a=1, b=0.0001 to 0.5, c=0.1 to 1.0 and d=0 to 1.0.

[0029] WO 98/47850 relates to a process for producing acetic acid fromethane, ethylene or mixtures thereof and a catalyst having the formulaW_(a)X_(b)Y_(c)Z_(d) in which X represents one or several of Pd, Pt, Agand Au, Y represents one or several of V, Nb, Cr, Mn, Fe, Sn, Sb, Cu,Zn, U, Ni, and Bi and Z represents one or several of Li, Na, K, Rb, Cs,Be, Mg, Ca, Sr, Ba, Sc, Y, La, Ti, Zr, Hf, Ru, Os, Co, Rh, Ir, B, Al,Ga, In, Tl, Si, Ge, Pb, P, As and Te, a=1, b>0, c>0 and d is 0 to 2.

[0030] WO 99/51339 relates to a catalyst composition for the selectiveoxidation of ethane and/or ethylene to acetic acid which compositioncomprises in combination with oxygen the elementsMo_(a)W_(b)Ag_(c)Ir_(d)X_(e)Y_(f) wherein X is the elements Nb and V; Yis one or more elements selected from the group consisting of Cr, Mn,Ta, Ti, B, Al, Ga, In, Pt, Zn, Cd, Bi, Ce, Co, Rh, Cu, Au, Fe, Ru, Os,K, Rb, Cs, Mg, Ca, Sr, Ba, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, Tl, U, Re andPd; a, b, c, d, e and f represent the gram atom ratios of the elementssuch that0<a<1, 0≦b<1 and a+b=1; 0<(c+d)≦0.1; 0<e≦2; and 0≦f≦2.

[0031] EP-A-1043064 relates to a catalyst composition for the oxidationof ethane to ethylene and/or acetic acid and/or for the oxidation ofethylene to acetic acid which composition comprises in combination withoxygen the elements molybdenum, vanadium, niobium and gold in theabsence of palladium according to the empirical formula:MO_(a)W_(b)Au_(c)V_(d)Nb_(e)Y_(f) wherein Y is one or more elementsselected from the group consisting of: Cr, Mn, Ta, Ti, B, Al, Ga, In,Pt, Zn, Cd, Bi, Ce, Co, Rh, Ir, Cu, Ag, Fe, Ru, Os, K, Rb, Cs, Mg, Ca,Sr, Ba, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, Tl, U, Re, Te, La and Pd; a, b,c, d, e and f represent the gram atom ratios of the elements such that:0<a≦1; 0≦b<1 and a+b=1; 10−5<c≦0.02; 0<d≦2; 0<e≦1; and 0≦f≦2.

[0032] WO 99/13980 relates to a catalyst for the selective oxidation ofethane to acetic acid of formula: Mo_(a)V_(b)Nb_(c)X_(d) wherein X is atleast one promoter element selected from the group consisting of P, B,Hf, Te and As; a is a number ranging from about 1 to about 5; b is 1; cis a number ranging from about 0.01 to about 0.5; and d is a numberranging from greater than 0 to about 0.1.

[0033] U.S. Pat. No. 5,300,682 relates to the use of oxidation catalystwith empirical formula of VP_(a)M_(b)O_(x) where M is one or more of Co,Cu, Re, Fe, Ni, Nb, Cr, W, U, Ta, Ti, Zr, Hf, Mn, Pt, Pd, Sn, Sb, Bi,Ce, As, Ag and Au, a is 0.5 to 3, b is 0 1 and x satisfies the valencerequirements.

[0034] U.S. Pat. No. 5,300,684 relates to a fluid bed oxidation reactionusing for exampleMo_(0.37)Re_(0.25)V_(0.26)Nb_(0.07)Sb_(00.03)Ca_(0.02)O_(x).

[0035] Other suitable oxidation catalysts for use in the presentinvention are described in WO 99/13980 which relates to the use ofcatalysts with elements in combination with oxygen in the relative gramatom ratios of MO_(a)V_(b)Nb_(c)X_(d) where X=P, B, Hf, Te or As; U.S.Pat. No. 6,030,920 which relates to the use of catalysts with elementsin combination with oxygen in the relative gram atom ratios of Mo_(a)V_(b)Nb_(c) Pd_(d); WO 00/00284 which relates to the use of catalystswith elements in combination with oxygen in the relative gram atomratios Of Mo_(a)V_(b)Nb_(c) Pd_(d) and/or Mo_(a) V_(b)La_(c)Pd_(d) ;U.S. Pat. No. 6,087,297 which relates to the use of catalysts withelements in combination with oxygen in the relative gram atom ratios ofMo_(a) V_(b)Pd_(c)La_(d); WO 00/09260 which relates to the use ofcatalysts with elements in combination with oxygen in the relative gramatom ratios of Mo_(a) V_(b)La_(c)Pd_(d)Nb_(e)X_(f) where X=Cu or Cr ande and f can be zero; WO 00/29106 and WO 00/29105 which relate to the useof catalysts with elements in combination with oxygen in the relativegram atom ratios of Mo_(a) V_(b)Ga_(c)Pd_(d) Nb_(e)X_(f) wherein X=La,Te, Ge, Zn, Si, In or W and WO 00/38833 which relates to the use ofcatalysts with elements in combination with oxygen in the relative gramatom ratios of Mo_(a) V_(b)La_(c)Pd_(d)Nb_(e)X_(f) wherein X=Al, Ga, Geor Si, the contents of which are hereby incorporated by reference.

[0036] Solid catalysts active for the oxidation of the C₂ to C₄ alkanemay be supported or unsupported. Examples of suitable supports includesilica, diatomaceous earth, montmorillonite, alumina, silica alumina,zirconia, titania, silicon carbide, activated carbon and mixturesthereof.

[0037] Solid catalysts active for the oxidation of the C₂ to C₄ alkanemay be used in the form of a fixed or fluidised bed.

[0038] The oxidation catalyst would be expected to oxidise at least partof any alkene fed to the oxidation reaction zone, for example to thecorresponding carboxylic acid.

[0039] The molar ratio of alkene to carboxylic acid produced in theoxidation reaction zone may be adjusted or maintained at apre-determined value by initially starting the reaction with a singlecatalyst and then replacing at least a part of the catalyst with atleast one other catalyst having a different selectivity to alkene andcarboxylic acid.

[0040] The molar ratio of alkene to carboxylic acid may be adjusted ormaintained by the replacement of at least a part of the catalyst in theoxidation reaction zone with one or more catalysts with selectivitiesdifferent to that of the catalyst or catalysts already present in theoxidation reaction zone. Thus, for example, the catalysts initiallypresent in the oxidation reaction zone may be overall more selective forthe production of alkene; the molar ratio of alkene to carboxylic acidproduced in the oxidation reaction zone, may then be adjusted byreplacement of at least a part of the catalysts already in the oxidationreaction zone with a catalyst or catalysts having a higher selectivityto carboxylic acid.

[0041] The catalyst or catalysts active for the oxidation of alkane toalkene and carboxylic acid may be replaced by methods known in the art.Thus, if operated in a fluid bed, catalyst may be removed either byentrainment or deliberately by known means and replaced by known means.

[0042] The molecular oxygen-containing gas used in the oxidationreaction zone may be air or a gas richer or poorer in molecular oxygenthan air. A suitable gas may be, for example, oxygen diluted with asuitable diluent, for example nitrogen or carbon dioxide. Preferably,the molecular oxygen-containing gas is oxygen. Preferably, at least someof the molecular oxygen-containing gas is fed to the oxidation reactionzone independently from the alkane and optional alkene feeds, and anyrecycle streams.

[0043] The alkane and, if used, alkene fed into the oxidation reactionzone of the process of the present invention may be substantially pureor may be admixed, for example, with one or more of nitrogen, methane,carbon dioxide, carbon monoxide, hydrogen, and low levels of C₃/C₄alkenes/alkanes.

[0044] Suitably, the concentration of optional alkene (as fresh feed andrecycle component) is from 0 to 50 mol % inclusive of the total feed,including recycles, to the oxidation reaction zone, preferably from 1 to20 mol %, more preferably from 1 to 15 mol %.

[0045] Suitably, the concentration of optional water (as fresh feed andrecycle component) is from 0 to 50 mol % inclusive of the total feed,including recycles, to the oxidation reaction zone, preferably from 0 to25 mol %.

[0046] When solid catalysts are used in the oxidation reaction zonealkane, optional alkene, molecular-oxygen containing gas and any recyclegases are preferably passed through the oxidation reaction zone with aresidence time corresponding to a combined gas hourly space velocity(GHSV) of 500-10,000 hr⁻¹; the GHSV being defined as volume (calculatedat STP) of gas passing through the reactor divided by the bulk volume ofsettled catalyst.

[0047] The oxidation reaction of the present invention may suitably becarried out at a temperature in the range from 100 to 400° C., typicallyin the range 140 to 350° C.

[0048] The oxidation reaction of the present invention may suitably becarried out at atmospheric or superatmospheric pressure, for example inthe range from 80 to 400 psig.

[0049] Typically, alkane conversions in the range 1 to 99% may beachieved in oxidation reaction of the present invention.

[0050] Typically, oxygen conversions in the range 30 to 100% may beachieved in the oxidation reaction of the present invention.

[0051] In the oxidation reaction of the present invention, the catalystsuitably has a productivity in the range 10 to 10000 grams of carboxylicacid, such as acetic acid, per hour per kilogram of catalyst.

[0052] Depending upon the nature of any catalyst used in any downstreamprocess, it is desirable that when used for the production of alkenylcarboxylate, such as vinyl acetate, the first gaseous product streamshould have a low concentration of carbon monoxide by-product as thismay have an adverse effect on some catalysts for the production ofalkenyl carboxylates e.g. vinyl acetate. Thus, it is preferred to use acatalyst in the oxidation reaction zone that gives negligible carbonmonoxide by-product. An additional catalyst component in the oxidationreaction zone may be used to oxidise carbon monoxide to carbon dioxide.This may be present in the oxidation catalyst or catalysts or in asecondary reaction zone.

[0053] When ethane is used as reactant for the oxidation process, theproduct stream comprises acetic acid, ethylene and water, and maycontain ethane and oxygen, inert gas components such as argon andnitrogen and the by-products, acetaldehyde, carbon monoxide and carbondioxide. Acetaldehyde and carbon monoxide may be converted by themolecular oxygen-containing gas to produce acetic acid and carbondioxide respectively, either in down stream processes or, afterrecycling, in the oxidation reaction zone. Ethylene is present in theproduct stream of the oxidation reaction as unconverted reactant ifethylene is present in the feed and/or as oxidation product of theethane reactant.

[0054] The product from the oxidation process may be fed directly orindirectly after one or more separation stages, to a second reactionzone together with optionally additional molecular oxygen-containinggas, optionally additional alkene and optionally additional carboxylicacid to produce alkenyl carboxylate, such as vinyl acetate. Carboxylicacid and/or alkene may be optionally recovered from the product of theoxidation process.

[0055] Unconverted alkane and/or alkene may be recycled together orafter at least partial separation from the downstream process to theoxidation reaction zone directly or indirectly after one or moreseparation stages.

[0056] Catalysts known in the art for the production of alkenylcarboxylates may be used in the process of the present invention. Thus,catalyst active for the production of vinyl acetate which may be used ina second reaction zone of the present invention may comprise, forexample, catalysts as described in GB 1 559 540; U.S. Pat. No. 5,185,308and EP-A-0672453 the contents of which are hereby incorporated byreference.

[0057] GB 1 559 540 describes a catalyst active for the preparation ofvinyl acetate by the reaction of ethylene, acetic acid and oxygen, thecatalyst consisting essentially of: (1) a catalyst support having aparticle diameter of from 3 to 7 mm and a pore volume of from 0.2 to 1.5ml/g, a 10% by weight water suspension of the catalyst support having apH from 3.0 to 9.0, (2) a palladium-gold alloy distributed in a surfacelayer of the catalyst support, the surface layer extending less than 0.5mm from the surface of the support, the palladium in the alloy beingpresent in an amount of from 1.5 to 5.0 grams per liter of catalyst, andthe gold being present in an amount of from 0.5 to 2.25 grams per literof catalyst, and (3) from 5 to 60 grams per liter of catalyst of alkalimetal acetate.

[0058] U.S. Pat. No. 5,185,308 describes a shell impregnated catalystactive for the production of vinyl acetate from ethylene, acetic acidand an oxygen containing gas, the catalyst consisting essentially of:(1) a catalyst support having a particle diameter from about 3 to about7 mm and a pore volume of 0.2 to 1.5 ml per gram, (2) palladium and golddistributed in the outermost 1.0 mm thick layer of the catalyst supportparticles, and (3) from about 3.5 to about 9.5% by weight of potassiumacetate wherein the gold to palladium weight ratio in said catalyst isin the range 0.6 to 1.25.

[0059] EP-A-0672453 describes palladium containing catalysts and theirpreparation for fluid bed vinyl acetate processes.

[0060] An advantage of using a palladium-containing catalyst is that anycarbon monoxide produced in the first reaction zone will be consumed inthe presence of oxygen and the palladium-containing catalyst in thesecond reaction zone, thereby eliminating the need for a separate carbonmonoxide removal reactor.

[0061] Typically, the production of alkenyl carboxylate such as vinylacetate in the second reaction zone is carried out heterogeneously withthe reactants being present in the gas phase.

[0062] Additional alkene reactant may be fed to the second reaction zonefor the production of alkenyl carboxylate as well as the alkene from theoxidation reaction zone as oxidation product and/or unconsumed alkenereactant.

[0063] Additional alkene introduced into the second reaction zone forthe production of alkenyl carboxylate may be substantially pure or maybe admixed, for example, with one or more of nitrogen, methane, carbondioxide, carbon monoxide, hydrogen, and low levels of C₃/C₄alkenes/alkanes.

[0064] The molecular oxygen-containing gas used in the second reactionzone for the production of alkenyl carboxylate may comprise unreactedmolecular oxygen-containing gas from step (a) and/or additionalmolecular oxygen-containing gas.

[0065] The additional molecular oxygen-containing gas, if used, may beair or a gas richer or poorer in molecular oxygen than air. A suitableadditional molecular oxygen-containing gas may be, for example, oxygendiluted with a suitable diluent, for example nitrogen or carbon dioxide.Preferably, the additional molecular oxygen-containing gas is oxygen.Preferably, at least some of the molecular oxygen-containing gas is fedindependently to the second reaction zone from the alkene and carboxylicacid reactants.

[0066] At least part of the carboxylic acid fed to the second reactionzone may be liquid.

[0067] When solid catalysts are used in the second reaction zone for theproduction of alkenyl carboxylate, the product from the oxidationreaction zone, any additional alkene or carboxylic acid reactants, anyrecycle streams and molecular oxygen-containing gas are preferablypassed through the second reaction zone at a combined gas hourly spacevelocity (GHSV) of 1000-10,000 hr⁻¹.

[0068] The second reaction zone for the production of alkenylcarboxylate may suitably be operated at a temperature in the range from140 to 200° C.

[0069] The second reaction zone for the production of alkenylcarboxylate may suitably be operated at a pressure in the range 50 to300 psig.

[0070] The second reaction zone for the production of alkenylcarboxylate may suitably be operated as either a fixed or a fluidisedbed process.

[0071] Carboxylic acid conversions in the range 5 to 80% may be achievedin the second reaction zone for the production of alkenyl carboxylate.

[0072] Oxygen conversions in the range 20 to 100% may be achieved in thesecond reaction zone for the production of alkenyl carboxylate.

[0073] Alkene conversions in the range 5 to 100% may be achieved in thesecond reaction zone for the production of alkenyl carboxylate.

[0074] In the second reaction zone for the production of alkenylcarboxylate, the catalyst suitably has a productivity in the range 10 to10000 grams of alkenyl carboxylate per hour per kg of catalyst.

[0075] When the alkane used in the process of the present invention isethane, the product stream from the second reaction zone for theproduction of alkenyl carboxylate may comprise vinyl acetate, water andacetic acid and optionally also unreacted ethylene, ethane,acetaldehyde, nitrogen, argon, carbon monoxide and carbon dioxide. Sucha product stream may be separated by azeotropic distillation into anoverhead fraction comprising vinyl acetate and water and a base fractioncomprising acetic acid and water. The base fraction may be removed fromthe distillation column as liquid from the bottom of the column, or as avapour one or more stages above the bottom of the column. Prior to sucha distillation step, ethylene, ethane, acetaldehyde, carbon monoxide andcarbon dioxide, if any, may be removed from the second product stream,suitably as an overhead gaseous fraction from a scrubbing column, inwhich a liquid fraction comprising vinyl acetate, water and acetic acidis removed from the base. The ethylene and/or ethane may be recycled tostep (a) and/or step (b).

[0076] Vinyl acetate is recovered from the overhead fraction, suitablyfor example by decantation. The recovered vinyl acetate may, if desired,be further purified in known manner.

[0077] The base fraction comprising acetic acid and water may berecycled, with or preferably without further purification, to step (b)of the process. Alternatively, acetic acid is recovered from the basefraction and may be further purified if desired, in known manner, forexample by distillation.

[0078] A suitable process for the production of esters by reaction ofthe carboxylic acid with the alkene is described in EP-A-0926126, thecontents of which are hereby incorporated by reference and which relatesto an esterification process comprising reacting in an addition reactiona lower olefin with a saturated lower aliphatic mono-carboxylic acid inthe vapour phase in the presence of a heteropolyacid catalystcharacterised in that the reaction is carried out in a plurality ofreactors set up in series such that the gases comprising the unreactedgases and products exiting from a first reactor are fed as the feed gasto a second reactor and those exiting from the second reactor are fed asfeed gas to the third reactor and so on for the subsequent reactors, andan aliquot of the reactant monocarboxylic acid is introduced into thefeed gas to each of the second and subsequent reactors so as to maintainthe olefin to monocarboxylic acid ratio in the feed gas to each of thesecond and subsequent reactors within a pre-determined range.

[0079] The invention will now be illustrated by way of example only andwith reference to the FIGURE and to the following examples.

[0080] The FIGURE represents in schematic block-diagram, apparatussuitable for use in the process of the present invention.

[0081] The apparatus comprises an oxidation reaction zone (1) providedwith a supply of ethane and optionally ethylene (3), a supply of amolecular oxygen-containing gas (4), a supply of recycle gas comprisingethane and optionally ethylene (5) and an outlet (18) for a firstproduct stream. Depending on the scale of the process, the oxidationreaction zone (1) may comprise either a single reactor or severalreactors in parallel or series.

[0082] The apparatus also comprises a second reaction zone (2) foracetoxylation of ethylene to vinyl acetate which is provided with means(17) for conveying at least a portion of the product from the firstreaction zone into the second reaction zone, a supply of molecularoxygen-containing gas (9), a supply of recycle acetic acid (10) and anoptional supply or supplies of ethylene and/or acetic acid (8).Depending on the scale of the process, the second reaction zone (3) maycomprise either a single reactor or several reactors in parallel or inseries.

[0083] The apparatus further comprises an optional scrubber (6) for thefirst reaction product; a scrubber (12) for the product from the secondreaction zone; means (13) for separating acetic acid from the product ofthe second reaction zone; vinyl acetate purfication means (14); optionalacetic acid purification means (15) and one or more separation means(16) for separating carbon dioxide from recycle gases from the secondreaction zone and optionally for recovery of ethylene product.

[0084] In use, the oxidation reaction zone (1) is provided with at leasttwo catalysts each active, but with different selectivities, for theoxidation of the ethane to form acetic acid and ethylene. Suitably theoxidation catalysts are solid catalysts. Molecular oxygen-containing gasis fed to the oxidation reaction zone (1) from supply (4) through one ormore inlets. A gaseous feedstock comprising ethane, and optionallyethylene is fed to the oxidation reaction zone (1) from supply (3).Recycle gas comprising ethane and ethylene is also fed to the oxidationreactor from supply (5). The molecular oxygen-containing gas, ethane andrecycle gas are introduced into the oxidation reaction zone through oneor more inlets separately or in partial or complete combination.Optionally, at least one of the streams fed to the oxidation reactoralso comprises water.

[0085] In the oxidation reactor a first product stream is produced whichcomprises ethylene (as product and/or unreacted feed), acetic acid,water, optionally unconsumed molecular oxygen-containing gas andby-products such as carbon monoxide, carbon dioxide, inerts andacetaldehyde. This may optionally be passed to a scrubber (16) fromwhich gas and liquid are removed. The gas may be recycled afterseparating by-products such as carbon dioxide and optionally recoveringethylene product by methods known in the art. Acetic acid may berecovered from the liquid, for example by distillation.

[0086] At least a portion of the first product stream is fed by means(17) into the second reaction zone which is provided with anacetoxylation catalyst, suitably a solid catalyst.

[0087] A molecular oxygen-containing gas is fed to the second reactionzone from supply (9). Acetic acid is fed to the second reaction zonefrom recycle supply (10). Optionally, additonal ethylene and/or aceticacid may be fed to the second reaction zone from supply or supplies (8).The first product stream, molecular oxygen-containing gas, recycleacetic acid and optional additional supplies of ethylene and/or aceticacid are fed into the second reaction zone through one or more inletsseparately or in partial or complete combination.

[0088] In the second reaction zone the ethylene, acetic acid andmolecular oxygen react to produce a second product stream comprisingvinyl acetate.

[0089] The second reaction product is passed to scrubber (12) from whichgas and liquid are separated. Carbon dioxide is separated from the gasand optionally ethylene product recovered, in one or more separationstages (16) by methods known in the art. The remaining ethane and/orethylene may be recycled to the first and/or second reactors. Aceticacid is separated from the scrubber liquid and is recycled to the secondreaction zone. Optionally, acetic acid product may be recovered from therecycle stream by means (15), for example by distillation. Vinyl acetateproduct is recovered from the scrubber liquid by means (14), for exampleby distillation.

[0090] In use, if one or more of the catalysts in the oxidation reactionzone de-activate and it is necessary to replace catalyst during theprocess, the molar ratio of ethylene to acetic acid produced may bemaintained at a constant, pre-determined value by controlling therelative proportions of the catalysts in the oxidation reaction zone.Thus, if the activity and/or selectivity of the catalysts in theoxidation reaction zone change independently during the process, thismay involve replacing at least part of the catalysts in the oxidationreaction zone by introducing catalysts into the oxidation reaction zonein different proportions to the proportions of catalysts in theoxidation reaction zone, in order to maintain the molar ratio ofethylene to acetic acid produced. Conversely, if the catalysts in thereactor deactivate such that their individual selectivities do notchange, it may be possible to maintain the molar ratio of ethylene toacetic acid produced by using for the replacement of catalysts in theoxidation reaction zone, catalysts in the same proportions as in theoxidation reaction zone.

[0091] Preferably, the molar ratio of ethylene: acetic acid produced inthe oxidation reaction zone is maintained at approximately 1:1, forexample, 0.8:1 to 1.4:1 for subsequent use in the second reaction zonefor the production of vinyl acetate. A different ratio may be maintainedif ethylene and/or acetic acid is separately recovered from theoxidation reaction product or separately added to the second reactionzone for the production of vinyl acetate. The molar ratio of ethylene toacetic acid may then be adjusted by controlling the relative proportionsof the at least two catalysts in the oxidation reaction zone, forexample to meet changes in market demand or feedstock availability.

[0092] Preparation of Catalysts Active for Ethane Oxidation (catalyst A)

[0093] A solution was prepared by dissolving 17.66 g ammonium molybdate,2.92 g ammonium vanadate, 3.24 g niobium chloride and 2.70 g oxalic acidin 400 ml water heated to 70° C. with stirring. To this solution wasadded 24.6 mg of ammonium tetrachloroaurate and 15.5 mg of palladiumacetate. After 15 minutes, the solution water was heated to boilingpoint followed by evaporation to dryness over 2 hours. The resultingcatalyst cake was ground and then calcined in static air in an oven at400° C. for 5 hours. The nominal empirical formula of the catalyst was

[0094] Mo_(1.00)V_(0.25)Nb_(0.12)Au_(0.0007)Pd_(0.0008)O_(x)

[0095] Preparation of Catalysts B-E

[0096] The method of preparation of catalyst A was repeated except thatthe gold-palladium component was replaced by a component selected fromthe group consisting of gold, copper, silver and phosphorus as shown inTable I below to give a range of catalyst compositions based upon a basecomposition having empirical formula Mo_(1.00)V_(0.25)Nb_(0.125)O_(x)but with different promoters. TABLE I Amount of compo- Nominal nentcatalyst Component precursor empirical Catalyst Precursor salt (g)formula Catalyst B Ammonium 0.428 gMo_(1.00)V_(0.25)Nb_(0.125)Au_(0.014)O_(x) tetrachloroaurate Catalyst CCopper acetate 0.280 g Mo_(1.00)V_(0.25)Nb_(0.125)Cu_(0.014)O_(x)Catalyst D Silver acetate 0.111 gMo_(1.00)V_(0.25)Nb_(0.125)Ag_(0.007)O_(x) Catalyst E Ammonium 0.090 gMo_(1.00)V_(0.25)Nb_(0.125)P_(0.0025)O_(x) hydrogen phosphate

[0097] Ethane Oxidation Reaction Method for Catalysts A-E

[0098] Typically, 5 ml of a powdered catalyst A-E was mixed with 15 mlof glass beads of diameter 0.4 mm to form a diluted catalyst bed of 20ml in volume. The diluted catalyst was then loaded into a fixed bedreactor made of Hastelloy of dimensions 12 mm internal diameter andlength 40 cm. The catalyst was maintained in position in the centre ofthe reactor using quartz wall plugs together with inert packing materialabove and below the catalyst bed. The apparatus was then pressure-testedat 20 bar with helium to check for leaks. The catalyst was thenactivated by heating to 220° C. at 5° C./min in helium at 21 bar for 4hours, to ensure full decomposition of catalyst precursors.

[0099] The required flows of ethane, 20% oxygen in helium and water werethen introduced to the reactor, to ensure the required inletcomposition. This composition was 42% v/v ethane, 6.7% oxygen, 25% v/vwater and balance helium. The total feed flow rate was maintained at alevel to ensure a feed GHSV of 2000-9000/h. After equilibrating for 60minutes, gas samples were taken from the outlet stream to a GC system(model Unicam 4400) to quantify ethane, ethylene, oxygen and helium.

[0100] The setpoint temperature of the reactor was increased until50-75% oxygen conversion was achieved, as indicated by the calculatedlevel of oxygen in the outlet stream. Following a further equilibrationperiod of 60 minutes, the catalyst was then evaluated under steady stateconditions for a period of typically 4-5 hours. Exit gas volume wasmeasured over the run period by a water-gas meter. Liquid products werecollected and weighed after the run period. Composition of gas andliquid products was measured using GC analysis (Unicam 4400 and 4200fitted with TCD and FID detectors respectively).

[0101] From analysis of the feed and product flow rates and compositionsthe following parameters were calculated:

[0102] ethane conversion=(inlet mol ethane—outlet mol ethane)/inlet molethane * 100

[0103] oxygen conversion=(inlet mol oxygen—outlet mol oxygen)/inlet moloxygen * 100

[0104] selectivity to acetic acid (C-mol %)=(outlet mol acetic acid *2)/(mol ethane converted * 2) * 100

[0105] selectivity to ethylene (C-mol %)=(outlet mol ethylene—inlet molethylene)* 2/(mol ethane converted * 2) * 100

[0106] selectivity to CO (C-mol %)=(outlet mol CO)/(mol ethaneconverted * 2) * 100

[0107] selectivity to CO₂ (C-mol %)=(outlet mol CO₂)/(mol ethaneconverted * 2) * 100 ethylene/AcOH ratio=(outlet mol ethylene—inletethylene mol)/(mol acetic acid) * 100.

[0108] STY (space time yield) %=(g acetic acid)/kg catalyst bed/hour

[0109] Typically, mass balance and carbon balance for a reaction wasfound to be 100+/−5%.

EXAMPLES 1 to 5

[0110] The catalysts A-E were employed in the general reaction methodabove. The results are shown in Table II below. TABLE II Oxidationreactions with base catalyst composition having empirical formulaMo_(1.00)V_(0.25)Nb_(0.125)O_(x) and with promoter components as inTable II Selectivity to Ethane Selectivity Selectivity carbon oxides,Molar ratio Catalyst conversion to ethylene to acetic acid COxEthylene:acetic STY to acetic acid Catalyst Component % % % % acidg/l-cat/h A Au—Pd 6.8 23.2 63.8 12.7 0.36:1 179 B Au 10.0 34.6 54.9 8.80.63:1 203 C Cu 8.0 35.8 55.6 8.3 0.64:1 158 D Ag 4.0 39.6 53.0 7.30.75:1 118 E P 12.2 56.9 28.3 9.6 2.01:1 130

[0111] The results of the above Examples show that the selectivities toethylene and acetic acid of the different catalysts under the samereaction conditions are different. Therefore, if at least two of thecatalysts are used in an oxidation reaction zone according to theprocess of the present invention, the molar ratio of ethylene to aceticacid can be adjusted or maintained at a predetermined value bycontrolling the relative proportions of the at least two catalysts inthe oxidation reaction zone.

[0112] Preparation of Catalysts F-N Active for Ethane Oxidation CatalystF

[0113] A solution ‘A’ was prepared by dissolving 107.70 g of ammoniummolybdate in 300 ml of distilled water heated to 70° C. with stirring. Asolution ‘B’ was prepared by dissolving 30.41 g of ammonium vanadate in300 ml of distilled water heated to 70° C. with stirring. A solution ‘C’was prepared by dissolving 18.91 g of niobium chloride 11.96 g ofantimony acetate, 2.76 g of potassium carbonate and 15.75 g of oxalicacid in 300 ml of distilled water heated to 70° C. with stirring. Eachof the solutions A, B and C was allowed to stand for 15 minutes to allowmaximum solubilisation of the reaction components. Solution C was thenrapidly added to solution B with stirring at 70° C. The solution B/C wasstirred for 15 minutes at 70° C. and then added rapidly to solution A.After 15 minutes the solution A/B/C was heated to boiling point followedby evaporation to dryness over 2.5 hours. The resulting dry catalystcake was then transferred to an oven for further drying at 120° C. for 2hours. After drying, the catalyst cake was ground to a fine powder. Theresulting powder was then sieved through a 0.2 mm mesh sieve. The sievedpowder catalyst was then calcined in static air in an oven at 400° C.for 4 hours. The nominal empirical formula of the catalyst was:Mo_(1.000)V_(0.426)Nb_(0.115)Sb_(0.066)K_(0.033)O_(x)

[0114] Catalyst G

[0115] A solution ‘A’ was prepared by dissolving 43.2 g of ammoniummolybdate in 100 ml of distilled water heated to 70° C. with stirring. Asolution ‘B’ was prepared by dissolving 11.4 g of ammonium vanadate in120 ml of distilled water heated to 70° C. with stirring. A solution ‘C’was prepared by dissolving 16.18 g of ammonium niobium oxalate and 2.5 gof oxalic acid in 100 ml of distilled water heated to 70° C. withstirring. Each of the solutions A, B and C was allowed to stand for 15minutes to allow maximum solubilisation of the reaction components.Solution C was then rapidly added to solution B with stirring at 70° C.After stirring solution B/C for 15 minutes at 70° C., solution A wasadded rapidly to it. After 15 minutes a solution ‘D’ (2.57 g of ammoniumphosphate dissolved in 20 ml water) was added with stirring. The A/B/C/Dsolution was heated to boiling point followed by evaporation to drynessover 1.5 hours. The resulting dry catalyst cake was then transferred toan oven for further drying at 120° C. for 16 hours. After drying, thecatalyst cake was ground to a fine powder. The resulting powder was thensieved through a 0.2 mm mesh sieve. The sieved powder catalyst was thencalcined in static air in an oven at 350° C. for 4 hours. The nominalempirical formula of the catalyst was:

Mo_(1.000)V_(0.400)Nb_(0.128)P_(0.080)O_(x)

[0116] Catalyst H

[0117] A solution ‘A’ was prepared by dissolving 22.935 g of ammoniummolybdate and 0.0357 g of ammonium gold chloride in 100 ml of distilledwater heated to 70° C. with stirring. A solution ‘B’ was prepared bydissolving 6.434 g of ammonium vanadate in 150 ml of distilled waterheated to 70° C. with stirring. A solution ‘C’ was prepared bydissolving 7.785 g of ammonium niobium oxalate in 100 ml of distilledwater heated to 70° C. with stirring. Each of the solutions A, B and Cwas allowed to stand for 15 minutes to allow maximum solubilisation ofthe reaction components. Solution C was then rapidly added to solution Bwith stirring at 70° C. The solution B/C was then stirred for 15 minutesat 70° C. and then added rapidly to solution A. After 15 minutes theA/B/C solution was heated to boiling point followed by evaporation todryness over 1.5 hours. The resulting dry catalyst cake was thentransferred to an oven for further drying at 120° C. for 2 hours. Afterdrying, the catalyst cake was ground to a fine powder. The resultingpowder was then sieved through a 0.2 mm mesh sieve. The sieved powdercatalyst was then calcined in static air in an oven at 400° C. for 4hours. The nominal empirical formula of the catalyst was:

Mo_(1.000)V_(0.423)Nb_(0.115)Au_(0.008)O_(x)

[0118] Catalyst I

[0119] A solution ‘A’ was prepared by dissolving 20.97 g of ammoniummolybdate and 0.0337 g of palladium acetate in 100 ml of distilled waterheated to 70° C. with stirring. A solution ‘B’ was prepared bydissolving 7.749 g of ammonium vanadate in 200 ml of distilled waterheated to 70° C. with stirring. A solution ‘C’ was prepared bydissolving 5.626 g of ammonium niobium oxalate, 0.598 g of antimonyacetate and 0.472 g of calcium nitrate in 200 ml of distilled waterheated to 70° C. with stirring. Each of the solutions A, B and C wasallowed to stand for 15 minutes to allow maximum solubilisation of thereaction components. Solution C was then rapidly added to solution Bwith stirring at 70° C. After stirring the solution B/C for 15 minutesat 70° C., solution A was rapidly added to it. After 15 minutes theA/B/C solution was heated to boiling point followed by evaporation todryness over 1.5 hours. The resulting dry catalyst cake was thentransferred to an oven for further drying at 120° C. for 2 hours. Afterdrying, the catalyst cake was ground to a fine powder. The resultingpowder was then sieved through a 0.2 mm mesh sieve. The sieved powdercatalyst was then calcined in static air in an oven at 350° C. for 4hours. The nominal empirical formula of the catalyst was:

Mo_(1.000)V_(0.5577)Nb_(0.0913)Sb_(0.0168)Ca_(0.0168)Pd_(0.0013)O_(x)

[0120] Catalyst J

[0121] A solution ‘A’ was prepared by dissolving 15.491 g of ammoniummolybdate in 100 ml of distilled water heated to 80° C. with stirring. Asolution ‘B’ was prepared by dissolving 5.594 g of ammonium vanadate and6.00 g of oxalic acid in 150 ml of distilled water heated to 80° C. withstirring. Each of the solutions A and B was allowed to stand for 15minutes to allow maximum solubilisation of the reaction components.Solution A was then rapidly added to solution B with stirring at 80° C.After stirring the solution A/B for 15 minutes at 80° C., 0.0053 g ofpalladium acetate and 0.0004 g of lanthanum nitrate were added withstirring. After 15 minutes the solution was heated to boiling pointfollowed by evaporation to dryness over 1.5 hours. The resulting drycatalyst cake was then transferred to an oven for further drying at 120°C. for 2 hours. After drying, the catalyst cake was ground to a finepowder. The resulting powder was then sieved through a 0.2 mm meshsieve. The sieved powder catalyst was then calcined in static air in anoven at 350° C. for 4 hours. The nominal empirical formula of thecatalyst was:

Mo_(1.000)V_(0.584)Pd_(0.000267)La_(0.00001)O_(x)

[0122] Catalysts K-N

[0123] Catalysts K-N were prepared by grinding together catalysts H(based on Au) and I (based on Pd) in different proportions. The relativequantities of catalysts H and I used to prepare each of catalysts K-Nare shown in Table III. TABLE III Catalyst Weight % catalyst H Weight %catalyst I Catalyst K 97.6 2.4 Catalyst L 95.2 4.8 Catalyst M 92.3 7.7Catalyst N 75.0 25.0

[0124] Ethane Oxidation Reaction Method for catalysts F-N.

[0125] Typically, 5 ml of a powdered catalyst F-N was mixed with 15 mlof glass beads of diameter 0.4 mm to form a diluted catalyst bed of 20ml in volume. The diluted catalyst was then loaded into a fixed bedreactor made of Hastelloy of dimensions 12 mm internal diameter andlength 40 cm. The catalyst was maintained in position in the centre ofthe reactor using quartz wall plugs together with inert packing materialabove and below the catalyst bed. The apparatus was then pressure-testedat 20 bar with helium to check for leaks. The catalyst was thenactivated by heating to 220° C. at 5° C./min in helium at 16 bar for 1hour, to ensure full decomposition of catalyst precursors.

[0126] The required flows of ethane, 20% oxygen in helium and water werethen introduced to the reactor, to ensure the required inletcomposition. This composition was 52% v/v ethane, 6.7% oxygen, 10% v/vethylene, 5% v/v water and balance helium. The total feed flow rate wasmaintained at a level to ensure a feed GHSV of 2000-9000/h, inparticular 3200/h. After equilibrating for 60 minutes, gas samples weretaken from the outlet stream to a GC system (model Unicam 4400) toquantify ethane, ethylene, oxygen and helium.

[0127] The setpoint temperature of the reactor was increased to 293° C.,to achieve a similar reactor temperature of 299-301° C. for each ofcatalysts F-J, in order to facilitate direct comparison. Following afurther equilibration period of 60 minutes, liquid product collectionwas commenced and continued for a period of typically 18 hours. Duringthe run period, the effluent gas composition was measured using GCanalysis (ProGC, Unicam). Exit gas volume was measured over the runperiod by a water-gas meter. The liquid products collected over the runperiod were recovered and weighed. The composition of the liquidproducts was measured using GC analysis (Unicam 4400 and 4200 fittedwith TCD and FID detectors respectively).

[0128] From analysis of the feed and product flow rates andcompositions, feed conversions, product selectivities, space time yield(STY) and the molar ratio of ethylene to acetic acid were calculatedusing the equations as given above under ethane oxidation reactionmethod for catalysts A-E.

EXAMPLES 6 to 10

[0129] The catalysts F to J were employed in the general reaction methodfor catalysts F to N above. The results are shown in Table IV below.TABLE IV Selectivity to Ethane Selectivity Selectivity carbon oxides,Molar ratio conversion to ethylene to acetic acid COx Ethylene:aceticSTY to acetic acid Catalyst Catalyst Component % % % % acid g/kg-cat/h FMo—V—Nb—Sb—K 4.5 12.9 59.3 27.8 0.22:1 97 G Mo—V—Nb—P 4.6 37.4 47.5 15.10.79:1 81 H Mo—V—Nb—Au 11.8 46.5 40.1 13.3 1.16:1 175 I Mo—V—Nb—Sb—Ca—Pd4.7 N/A^(a) 76.0 24.0 −0.54:1^(b)   216 J Mo—V—Pd—La 4.6 N/A^(a) 71.428.7 −0.55:1^(b)   201

[0130] The results of Examples 6-10 demonstrate that the selectivitiesto ethylene and acetic acid of the different catalysts under the samereaction conditions are different and thus the molar ratio of ethyleneto acetic acid could be adjusted or maintained at a pre-determined valueby using two different catalysts in the oxidation reaction zone incontrolled proportions.

EXAMPLES 11-14

[0131] The mixed catalysts K to N were employed in the general reactionmethod for catalysts F to N above. The results are shown in Table Vbelow. TABLE V Selectivity to Catalyst H and I Ethane SelectivitySelectivity carbon oxides, Molar ratio proportions conversion toethylene to acetic acid COx Ethylene:acetic STY to acetic acid Catalystwt % % % % % acid g/kg-cat/h H Mo—V—Nb—Au 11.8 46.5 40.1 13.3 1.16:1 175K 97.6 H—2.4 I 8.5 36.5 47.5 16.0 0.77:1 149 L 95.2 H—4.8 I 7.0 8.2 66.425.4 0.12:1 172 M 92.3 H—7.7 I 6.2 N/A^(a) 73.7 26.3 −0.08:1^(b)   179 N75.0 H—25.0 I 5.0 N/A^(a) 75.8 24.2 −0.41:1^(b)   203 I Mo—V—Nb—Sb—Ca—4.7 N/A^(a) 76.0 24.0 −0.54:1^(b)   216 Pd

[0132] Inspection of Table V clearly illustrates that by controlling therelative proportions of the different catalysts H and I in the oxidationreaction zone the molar ratio of ethylene to acetic acid can be adjustedto a pre-determined value.

What is claimed is:
 1. A process for the oxidation of a C₂ to C₄ alkaneto produce the corresponding alkene and carboxylic acid which processcomprises contacting in an oxidation reaction zone, said alkane,molecular oxygen-containing gas, and optionally, at least one of thecorresponding alkene and water, in the presence of at least twocatalysts each active, with different selectivities, for the oxidationof the alkane to the corresponding alkene and carboxylic acid, toproduce a product stream comprising said alkene, carboxylic acid andwater, and in which process the molar ratio of alkene to carboxylic acidproduced in said oxidation reaction zone is adjusted or maintained at apre-determined value by controlling the relative proportions of the atleast two catalysts in said oxidation reaction zone.
 2. An integratedprocess for the production of an alkyl carboxylate which processcomprises the steps: (a) contacting in an oxidation reaction zone a C₂to C₄ alkane, a molecular oxygen-containing gas and optionally, at leastone of the corresponding alkene and water in the presence of at leasttwo catalysts each active, with different selectivities, for theoxidation of the alkane to the corresponding alkene and carboxylic acid,to produce a product stream comprising alkene, carboxylic acid andwater; and (b) contacting in a second reaction zone at least a portionof each of said alkene and carboxylic acid produced in the firstreaction zone, in the presence of at least one catalyst active for theproduction of alkyl carboxylate to produce said alkyl carboxylate, andin which the molar ratio of alkene to carboxylic acid produced in theoxidation reaction zone is adjusted or maintained at a pre-determinedvalue by controlling the relative proportions of the at least twocatalysts in said oxidation reaction zone.
 3. An integrated process forthe production of an alkenyl carboxylate which process comprises thesteps: (a) contacting in an oxidation reaction zone a C₂ to C₄ alkane, amolecular oxygen-containing gas and optionally, at least one of thecorresponding alkene and water in the presence of at least two catalystseach active, with different selectivities, for the oxidation of thealkane to the corresponding alkene and carboxylic acid, to produce aproduct stream comprising alkene, carboxylic acid and water; and (b)contacting in a second reaction zone at least a portion of each of saidalkene and carboxylic acid produced in the first reaction zone and amolecular oxygen-containing gas, in the presence of at least onecatalyst active for the production of alkenyl carboxylate to producesaid alkenyl carboxylate, and in which the molar ratio of alkene tocarboxylic acid produced in the oxidation reaction zone is adjusted ormaintained at a pre-determined value by controlling the relativeproportions of the at least two catalysts in said oxidation reactionzone.
 4. A process as claimed in any one of the preceding claims inwhich the molar ratio of alkene to carboxylic acid produced in theoxidation reaction zone is in the range 1:10 to 10:1.
 5. A processaccording to claim 4 wherein the molar ratio of alkene to carboxylicacid produced in the oxidation reaction zone is in the range 0.8:1 to1.4:1.
 6. A process as claimed in claim 2 or claim 3 in which the alkeneand/or carboxylic acid is separately recovered from the oxidationreaction product or separately added to the second reaction zone.
 7. Aprocess as claimed in any one of claims 1 to 3 in which the alkane isethane, the corresponding alkene being ethylene and the correspondingcarboxylic acid being acetic acid.
 8. A process according to claim 2wherein the alkyl carboxylate is ethyl acetate
 9. A process according toclaim 3 wherein the alkenyl carboxylate is vinyl acetate
 10. A processaccording to claim 8 or claim 9 and wherein the molar ratio of alkene tocarboxylic acid produced in the oxidation reaction zone is in the range0.8:1 to 1.4:1.
 11. A process according to any one of claims 1 to 3wherein the molar ratio of alkene to carboxylic acid produced in theoxidation reaction zone is adjusted or maintained at a pre-determinedvalue by replacing at least part of the catalyst(s) present in theoxidation reaction zone with one or more catalysts with selectivities toalkene and/or carboxylic acid different to that of the catalyst(s)present in the oxidation reaction zone.
 12. A process according to anyone of claims 1 to 3 in which at least one of the at least two catalystsin the oxidation reaction zone comprises molybdenum.
 13. A processaccording to claim 12 wherein each catalyst in the oxidation reactionzone comprises molybdenum
 14. A process according to claim 3 or claim 9in which the catalyst present in the second reaction zone comprisespalladium.
 15. A process according to claim 3 or claim 9 in whichadditional alkene is fed to the second reaction zone as well as thealkene from the oxidation reaction zone.
 16. A process according toclaim 3 or claim 9 in which additional molecular oxygen-containing gasis fed to the second reaction zone as well as the unreacted molecularoxygen-containing gas from the oxidation reaction zone.